The present invention relates generally to position measuring devices and more specifically to position measuring devices which include a measurement embodiment which is scanned at multiple locations.
It is known in the art that position measuring devices provide more accurate measurements if the position measurement embodiment of the measuring device is scanned at multiple scanning locations with multiple scanning devices.
Such position measuring devices are taught for example, in U.S. Pat. Nos. 4,580,046 and 4,580,047. Each of these two references discloses an arrangement in which a rotational drive is scanned at two locations and in which the velocity of the components to be measured and the phase displacement between the two measuring signals is monitored by a timed testing circuit.
German DE-A1-35 27 128 also discloses a rotational measuring device which includes two scanning locations. In the arrangement disclosed by this reference, the amplitude of the measuring signals generated at the scanning locations is kept constant. To implement the double-scanning, the rotational drive is provided with slit gratings which are scanned by two signal generators which are offset spatially by 180.degree. from each other. Each signal generator generates two sine voltage signals offset by 90.degree. relative to one another. The angle of rotation of the rotational drive is determined from an average of these two signals.
Another double-scanning arrangement is taught by German DE-A1-27 11 593 which discloses an angular displacement measuring device which includes two sets of slit gratings on an interruptor wheel. The sets of gratings are disposed such that each set is 180.degree. apart from the other on the interruptor wheel. The gratings are scanned by two signal generators which are also spatially offset by 180.degree.. The output signals in this arrangement are also averaged to provide a measurement value.
These double-scanning arrangements reduce measurement inaccuracies which are caused by eccentricity tolerances and mechanical graduation errors. In such double-scanning arrangements, phase displacements between the scanning signals provided from the two scanning locations can be tolerated to a certain degree. However, if the phase displacement exceeds a set limit, the resulting signals may become too small or the signals may even mutually cancel each other entirely. This condition is explained in more detail below.
The danger of exceeding the displacement limit is most prevalent if these double-scanning devices are disposed in a location which is subjected to high accelerations such as accelerations caused by impacts or strong vibrations. Under these conditions, the double scanning--which normally provides improved measuring accuracy--is actually disadvantageous since the signals from a first scanning location can either partially or entirely cancel the signals from the other scanning location leading to erroneous measurements.
Therefore, in view of the above, it is an object of the present invention to provide a position measuring apparatus which will under normal conditions advantageously use multiple scanning to provide accurate measurements.
It is another object of the present invention to provide a position measuring apparatus which incorporates multiple scanning locations and which will provide reliable measurement readings even when used on devices subjected to shocks or other jarring motions.